1. Field of the Invention
The present invention relates in general to an apparatus for detecting a suction air flow rate necessary for controlling an internal combustion engine and in particular to an air flow meter for detecting a suction air flow rate by utilizing Karman' vortex generation.
2. Description of the Prior Art
As the statutory restrictions on the exhaust gases of internal combustion engines becomes severe, there arise many problems in the carbureter in respect of the controllability of ratio of air and fuel in a combustible mixture, distribution of the fuel mixture to combustion chambers, changes in properties of air-fuel mixture as a function of time, heat resistance property or the like, which makes more difficult the processing of the exhaust gas. Under the circumstance an electronic fuel injection or jetting apparatus has come into practical use which significantly avoids the above disadvantages and is adapted to control the fuel flow independence on detected operating states of an associated internal combustion engine with the aid of parameters such as aperture of a throttle valve, negative suction pressure and number of revolutions of an engine. At present, such electronic fuel jetting injection is gaining a large share of the commercial field and is increasingly employed in place of the stationary carbureter of Venturi type. However, since expensive parts are required in the fuel injection valve and the detector device of such electronic fuel injection, high manufacturing costs thereof are involved as compared with the stationary Venturi type carbureter. Accordingly, application of the electronic fuel injection apparatus is at present restricted to a specifical type of motor vehicles such as high class motor cars.
In the light of the fact that expensive sensors are required for detecting the throttle valve aperture, negative suction pressure and the number of revolutions the engine as parameters representative of the operating state of engine, there has been developed another type of electronic fuel injection system which is adapted to detect directly the suction air flow. For example, it is known that a movable air valve which is mounted in an air feeding conduit and is caused to be moved in response to variation in the air flow rate and vary the resistance value of a variable resistor, thereby to produce an electric output signal which is compatible to the air flow rate. Further, a fuel injection system is also known in which a Karman' vortex type flow meter is utilized. In the former case, however, a high accuracy can not be obtained in the operation since the movable mechanical valve is relatively less sensitive to the variation in the air flow rate. Besides, there arises the possibility that the air valve should be demaged due to the back-fire phenomenon which occurs in the starting operation in the cool state of engine. Further resistance to air suction may be increased, involving decreased output of engine. On the other hand, the system in which the Karman' vortex type flow meter is used is considered advantageous in that the number and the state of Karman' vortexes as produced are little influenced by temperature, density, viscosity of air flows and the pressure loss is of a negligible order. Thus, the Karmans' vortex type flow meter is excellent in respect of the accuracy and reproducibility. For the generation of the Karman' vortexes, the following equation will validly apply. EQU f = St. (V/D) (1)
where
f: frequency at which the vortexes are generated,
St: Strouhal number,
V: velocity of air flow, and
D: diameter of column.
As shown in FIG. 1, the Strouhal number St is substantially constant at about 0.2 when the velocity of suction air flow is in the range of 3 m/sec to 80 m/sec. The range of air flow velocity in which the Strouhal number remains at a constant value will of course be varied in dependence on the geometrical configurations of the vortex generator. In general, the ratio between the Strouhal numbers at the maximum and the minimum flow velocities is on the order of 30 to 40. In the regions outside the range in which the Strouhal number St is constant, the vortex signal becomes extremely unstable and degraded in accuracy for use as the flow velocity signal. On the other hand, in an internal combustion engine which is operated in a normal state, the ratio between the quantities of suction air at the maximum and the minimum flow rates (corresponding to points O' and O in FIG. 2, respectively) is on the order of 50 to 60. Thus, it is difficult to measure the flow rate with an acceptable high accuracy by means of a single Karman flow meter over the whole operation range of the engine.